The following invention relates to a glass break detector and more particularly to an acoustic sensing device that senses two different frequency characteristics of breaking glass and provides an alarm upon the detection of both occurrences within preselected time frames. The invention results from the discovery that breaking glass produces highly characteristic patterns of acoustic waves, and in particular, produces a characteristic positive low frequency acoustic wave and a high frequency set of acoustic waves that follow the initial low frequency phenomenon.
In the past, glass break detectors have attempted to eliminate the occurrence of false alarms by focusing on high and low frequency characteristics of breaking glass. The U.S. Pat. No. 4,091,660, to Yanagi, detects signals in a frequency range of less than 50,000 cycles and of greater than 100,000 cycles, producing an enabling signal for an alarm when both frequency components are present at the same time. Other devices recognize that different frequency components may be present at different times. In Davenport et al. U.S. Pat. No. 4,668,941, it is presumed that breaking glass produces an initial low frequency thump centered around 350 Hz followed by a high frequency component centered at around 6.5 kHz. The 6.5 kHz signal is indicative of glass that breaks as it falls on the floor and shatters producing a tinkling sound. But as pointed out in Abel et al. U.S. Pat. No. 4,837,558, one can not always presume that glass once broken will produce the tinkling sound, particularly if the glass pane or window is situated above a carpet in an office or residence.
For some time intrusion detectors have made use of the phenomenon that the opening of a door or window produces an infrasonic pressure wave that may be detected by a sensitive microphone or other acoustic transducer having a frequency response in the region of one to five or ten cycles per second. An example of such a device is shown in Yarbrough et al. U.S. Pat. No. 4,853,677. The Yarbrough device also includes a glass break detector circuit that is coupled to the same microphone. Either a high frequency or a low frequency event will trigger an alarm if either produces the appropriate frequency spectrum. Furthermore, it has been recognized that the opening of a door or window produces negative-going air pressure in the first instance and acoustic detectors which are intrusion detectors have been designed to take advantage of this fact. An example is shown in Goldstein et al. U.S. Pat. No. 4,991,145.
The aforementioned glass breakage and intrusion detectors take advantage of some of the characteristics of breaking glass but do not always inhibit false alarms which may be produced by events that have frequency characteristics similar to those produced by breaking glass. Moreover they fail to take into account the fact that, especially in the low frequency region, different types of glass emit different frequency spectra when they break.